Tunable semiconductor laser diodes (TSLD's) are key components in the implementation of present and future optical communication systems. The first TSLD's used temperature control tuning, i.e. a temperature change generated in the TSLD caused a change in its physical dimensions, thus changing its wavelength. This method of tuning the laser is relatively slow. The tuning time is in the order of milliseconds or more, making it too slow for applications where fast wavelength switching is required. Furthermore, the repeatability, accuracy and tuning range are limited and unsuitable for advanced applications such as dense dynamic WDM systems, and optical switches and routers.
A laser can radiate at wavelengths that have a round trip phase change through the cavity from end to end of 2kπ (k in an integer). These wavelengths define the Fabry Perot modes of the cavity. In order to ensure single longitudinal mode operation of the laser, the net gain of the preferred lasing mode must be higher then that of the other Fabry Perot modes. In a TSLD, cavity mode discrimination is achieved using wavelength selective structures such as reflectors, sampled reflectors (SG), super structure reflectors (SSG) and couplers.
A new family of TSLD's have recently been developed, which use the free plasma effect in these structures as the tuning mechanism. By injecting current into these structures a change in the optical refraction index is invoked, thus changing their optical properties and ultimately changing the wavelength of the laser.
When the laser is correctly tuned, all of these sections provide the maximal transmission at the desired wavelength and attenuate other modes, thus ensuring single mode operation. If the laser is not correctly tuned, i.e. the wavelength selective sections are not all aligned to a common wavelength, or the phase current is not adjusted to meet the phase condition at the desired wavelength, the laser may operate in an unstable mode, or may not operate at all. An arbitrary set of tuning currents usually results in no lasing power. This is why a multi-dimensional scan of the tuning currents generally yields comparatively poor tuning results.
These TSLD's provide fast tuning capabilities together with accuracy, stability and repeatability. The number of structures or sections in such a laser determine the number of input currents that have to be provided in order to tune the laser.
Characterization of a tunable, multiple section semiconductor laser is a process in which the tuning currents (or voltages) necessary to tune the laser to a given wavelength are determined. The term characterization is thuswise used and claimed in this application. The characterization procedure presents a serious problem in laser manufacture. This information can not be determined during production, and is different for each individual laser due to microscopic differences between the lasers, even though manufactured to be nominally identical.
The difficulty in measuring the spectral response and tuning characteristic of a particular tuning section of a laser is that the overall response of the laser depends at the same time on all the other sections. When changing the tuning current in a certain section in order to measure the shift in the response of that section, the laser power usually drops unless adjustment is also made to the other currents.
Current techniques for the characterization of multiple section lasers, such as DBR, SG-DBR, and GCSR lasers, use methods largely based on trial and error. A set of currents is introduced to the laser, and the wavelength and optical power are measured. The set is slightly changed and the process is repeated. As there are generally three or four different tuning currents, depending on the number of sections, this process involves a threeor four-dimensional scan of the input currents to the laser. In this way the tuning currents for different wavelengths can be obtained. This method is very time and labor consuming, currently taking several hours per laser, and adds significant additional costs to the laser beyond the direct manufacturing costs. More recent developments of this basic method use what could be termed as intelligently directed trial and error methods, but the characterization procedure is still a time consuming process. A recent method is described in the PCT Patent Application by B. Broberg et al, for “Method of optimizing the operation points of lasers and means for carrying out the method”, published as No. WO 99/40654 and hereby incorporated by reference in its entirety. Another recent method of laser characterization is given in the article entitled “Novel mode stabilization scheme for widely tunable lasers” by G. Sarlet et al, hereby incorporated by reference in its entirety, published in the Proceedings of the European Conference on Optical Communications (ECOC) 1999, pp. 128–129, in which the authors describe a method of characterizing an SSG-DBR laser, in which a two-dimensional scan of the front SSG and rear SSG is employed.
There therefore exists an urgent need for a method of characterization of TSLD's, which can be performed simply, speedily, and at low cost, to enable more cost-effective, widespread use of these devices.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.